This invention, in its preferred form, relates to meter provers for testing the accuracy of fluid and in particular gas meters.
In the prior art, U.S. Pat. No. 185,319 of Harris is an early example of the use of a bell-type meter prover comprising a bell-shaped container or bell that is rectilinearly moved into and from a container or kettle filled with a liquid such as oil. Typically, a pulley arrangement is used whereby a pulley is located above the bell, with a cord suspended about the pulley having one end attached to the bell and the other end to a set of weights. A conduit is provided from the bell to the meter to be tested, whereby as the bell is drawn upward, a fluid, e.g., gas, is drawn through the meter and into the bell. A valve is placed within the conduit and when disposed to its closed position, prevents the flow of the fluid from the meter into the space defined by the bell and its kettle, thus inhibiting the motion of the bell and the weight suspended therefrom by the pulley. Upon opening of the valve, the fluid flows into the bell permitting the weights to exert its force upon the bell, thereby lifting the bell. When the weights are released, the cord and thus the bell are pulled upward, thus creating a vacuum within the bell, the oil providing a seal to prevent leakage of air otherwise into the bell.
In order to determine the amount of fluid that is drawn through the meter, the early practice sought to control the extent of movement of the bell, correlating this movement to a given quantity of fluid that would be drawn through the meter and comparing the known quantity of fluid drawn through the meter and into the meter prover, to the fluid as measured by the meter, typically indicated by the meter's dial positions. Current methods require physical measurements of the dimensions of the bell (bell strapping) which are inconvenient and are subject to a number of possible errors incurred by averaging the non-uniform geometrical diameters and the wall thicknesses of the bell and by interpolating the scale markings by eye. The quoted accuracy of such current methods is about 0.3% at best. Thus, it can be seen that such a bell-type meter prover, which was the calibrating standard for fluid meters, lacked inherently a high degree of accuracy due to the errors introduced by (1) the visual sightings of the beginning and final points of the bell movement, (2) the visual sightings of the initial and terminating volume indications by the meter dial, and (3) the inherent inaccuracy of determining the volume of the bell. The most significant cause of error in this technique was due to the difficulty of accurately measuring and determining the volume of the bell. The bell, itself, was formed with as great an accuracy as possible, but variations in its diameter, and therefore circumference, inherently occurred. The taking of many measurements of the circumference by bell strapping was the best method then devised to obtain the bell's average circumference and therefrom the volume of the cylindrical portion of the bell.
The use of the bell-type prover has persisted for many years with improvements being made thereto primarily in the nature of determining the movement of the bell and in determining the volume of fluid passed through the meter. One of the earliest examples of an automated prover system is found in U.S. Pat. No. 3,050,980, of Dufour et al., which discloses a bell having optical pick offs to sense the movement of its bell as it is directed upwardly. A conduit is directed from the bell to the meter having a first solenoid actuated valve for controlling the flow of fluid from the meter to the bell, as well as a second solenoid actuated valve coupled to the conduit for permitting discharge of the fluid from the bell as it returns to its downmost position. In operation, the bell, initially filled with air, is lowered into its tank tending to drive air through the meter. A dial hand on the meter register, known as the "prover hand" is detected by means of an optical pick-up to initiate the test, whereby the first valve is opened, while maintaing the second valve closed, to permit a flow of the fluid from the bell through the meter. An automatic airtight test is described wherein both the first inlet and second discharge valves are closed, and as pressure is built up, tests are made for leaks in the system and its valves by measuring the pressure established within the bell.
Further, U.S. Pat. No. 2,987,911 of McDonell suggests a prover system in which first and second temperature sensors are disposed at the outlets of the meter and of the prover, respectively, whereby the temperature differences is calculated to develop a temperature compensation factor Tc, which is used to make a correction in the calculated volume.
As suggested by U.S. Pat. No. 3,933,027 of Mehall, efforts were made to improve the bell-type prover system by automating its operation. The Mehall patent '027 suggests the placement of a series of sensing flags with respect to its bell, whereby an optical encoder senses the movement of these flags to provide indications of corresponding volumes of air as drawn by the bell's prover through the coupled meter. Further, a second optical encoder is coupled to the dial of the meter to provide an output as a train of pulses indicative of the volume flowing through the meter. At initiation of the meter test, a gate is activated by the first optical encoder to initiate a counting or timing procedure whereby a clock signal is applied to each of a bell clock counter and a meter clock counter. The gate passing the clock signals to the bell counter is disabled upon reaching a given count corresponding to a known quantity of fluid as drawn through the meter. When a similar quantity of fluid has been measured by the meter, as indicated by the second optical encoder, a signal therefrom is applied to a gate to terminate the application of clock signals to the meter clock counter. At termination, first and second counts have been accumulated within the bell clock and meter clock counters, whereby the ratio thereof may be readily calculated and displayed upon a suitable digital display. This ratio is understood to be the meter registration, i.e., the ratio of the actual or calibrated volume of fluid passed through the meter to that measured by the meter.
U.S. Pat. No. 3,937,048 of St. Clair et al. provides similar teachings to the Mehall patent '027 disclosing a bell-type automatic meter prover wherein there is further included a device for sensing the series of pulses produced by the meter during a cycle of its operation. The volume actually passed through the meter is measured by an encoder which produces a train of pulses indicative of the linear movement of the bell and therefore the volume displaced into or out of the bell during a test. The encoder provides a train of pulses indicative of the volume displaced by the bell; the encoder pulses are accumulated for a given number of meter operation cycles, to calibrate the meter indication of volume with a known volume of fluid displaced by the bell.
U.S. Pat. No. 3,877,287 of Duntz, Jr. suggests a substantially different structure, wherein in place of the bell-type container, a cylinder is used to receive a piston driven through the cylinder at a controlled rate by a motor rotatively coupled by a lead screw to the piston for driving it through the cylinder as the motor rotates. As a result, the piston is driven at a constant velocity through the precision bore tube or cylinder to drive fluid from the cylinder and through the meter to be tested. The Duntz, Jr. patent '287 suggests two ways of measuring the fluid flow rate, the first involving placing a series of holes in a piston rod interconnecting the piston and the lead screw, and sensing the movement of the holes past a photodetector. A second method uses an optical encoder coupled to the drive motor to provide an output train of pulses indicative of piston displacement and therefore the actual fluid volume displaced from the cylinder.
U.S. Pat. No. 3,631,709 of Smith also discloses a meter prover comprising a piston and cylinder arrangement, wherein the piston is driven via a lead screw by a program controlled motor. Upon actuation, the motor drives via the lead screw and piston through the cylinder, whereby a known volume of fluid (water) is drawn through a series of meters disposed in series. The control program of the motor causes the piston to move at different rates of speed, whereby corresponding fluid flow rates are established through the meters for a single stroke of the piston through the cylinder. A magnet is coupled to a shaft interconnecting the lead screw and the piston to actuate a reed switch as the piston is drawn through the cylinder, to initiate the counting of pulses derived from a first or master pulser coupled to the motor. The output of the master pulser is a train of pulses and is applied to a register to provide an indication of the actual flow through the meters. Optical encoders are also coupled to each of the meters to provide pulse signals to a second set of registers whereby the measured values of fluid flow measured by the meters may be accumulated and displayed. The standard or actual volume of flow is defined as a specific number of counts from the first or master pulser against which the output from the individual meters is compared. The program control of the motor permits the acceleration of the motor and its piston to a steady state condition before beginning measurement of the fluid flow through the meter in order to permit any transients in the fluid to settle.
As indicated above, the prior art has dealt with providing automation to the process of testing meters by automatically initiating and terminating the counting of pulses from a first encoder indicative of the standard volume of fluid drawn through the meter, as well as the counting of pulses from a second encoder indicative of the volume of fluid measured by the meter under test.
However, the prior art has not dealt with the problem of improving the basic accuracy of the meter prover, i.e., the basic meter calibrating device. At this point in the development of the art, meter provers, particularly those of the bell-type, are only able to achieve an accuracy of .+-.0.2% under optimum conditions. It is thus obvious that the fluid meters calibrated or tested with such provers may achieve no greater accuracy themselves. One of the primary reasons for the lack of ultimate precision in existing meter provers, is the lack of precise methods of and apparatus for measuring with high precision the volume displaced within either the bell or the cylinder as disclosed by the above-discussed patents.
It is contemplated by this invention to provide a method and apparatus for measuring the volume of the test chamber of the meter prover with an accuracy to one part in 10.sup.6. Once the volume can be obtained with such accuracy, then it is necessary to insure, as taught by this invention, that the structure containing the chamber is rigid and nondeformable. In the past, the bell-type enclosures have not provided such a rigid structure so that if they were accidentally jarred, the interior volume may be changed to a degree to affect the accuracy of the bell-type meter prover's readings. As will be disclosed, this invention adopts a technique for measuring the displacement volume within the meter prover's chamber by generating electromagnetic waves and determining the frequency at which resonance is established at first and second positions of a piston to be driven through the housing. Adopting such a method of measuring the container's volume requires, as taught by this invention, the use of a housing having a substantially perfect right circular cylinder configuration so that the frequencies at which resonance is established, may be determined sharply to thereby determine the displacement volume within the rigid cylinder. Further, in the development of the invention to be described, it became evident that once the volume of the prover had been determined with great accuracy, then it was necessary to determine other parameters as would affect the indication of meter registration or of the volume of fluid as drawn through the meter under test, with similar accuracy. In this regard, the invention contemplates methods and apparatus for measuring with a high degree of accuracy the temperature and pressure of the fluid within the meter and within the meter prover such that a correction factor may be determined with a similar degree of accuracy to thereby correct for variations in these parameters that exist in the meter under test and the meter prover to this invention. Such a technique contrasts to the prior art, wherein a meter prover was put into an environmentally conditioned room with limited variations in the temperature and pressure of that room. However, when the precision with which the variables are to be measured approaches 10.sup.6 as has been achieved by this invention for the measurement of the displacement volume of the meter prover, then it becomes necessary to note that these parameters of pressure and temperature do vary within the meter prover and within the fluid flow meter during the course of its test such that to insure desired precision, that new methods and apparatus for measuring pressure and temperature must be provided. For example, if there is an error of 1.degree. F. in the measurement of fluid temperature, there may result an error of 0.2% in the displacement volume indicated by the prover. It is contemplated that the meter prover system of this invention is capable of achieving an indication of the volume as drawn through the meter under test to a precision of 0.004%. With such accuracy, then investigations may be conducted to determine the effects of other factors upon the measurement of fluid flow. For example, the number of times that tests are performed upon a given meter will affect the measured meter registration. Further, it is contemplated that the variation in the rate of fluid flow as well as the volume of fluid flow through the meter will affect the indicated measured volume by the meter as well as its meter registration with respect to its standard volume as measured by a meter prover.